UV curable coatings are one of the fastest growing sectors in the coatings industry. In recent years, UV technology has made inroads into a number of market segments like fiber optics, optical- and pressure sensitive adhesives, automotive applications like UV cured topcoats, and UV curable powder coatings. The driving force of this development is mostly the quest for an increase in productivity of the coating and curing process. In automotive refinish applications where minor repairs need to be performed swiftly and at ambient temperature, UV technology promises to significantly increase the throughput of cars in a body shop. The development of refinish applications breaks new ground in UV technology. Safety concerns associated with the use of UV lamps in body shops as well as economic constraints will likely preclude the use of high intensity light sources. Relatively inexpensive low intensity lamps that emit only in the UV-A region of the electromagnetic spectrum are taking their place thus posing new challenges to resin developers and formulators.
UV curable coating compositions are known in the art. U.S. Pat. No. 5,684,081 describes a radiation-curable, aqueous dispersion, although the reference is silent as to the wavelength of the radiation to be used. Also known are compositions that are curable using UV radiation having a very low UV-B content and substantially no UV-C content (see, e.g., U.S. Patent application publication 2003/0059555 and U.S. Pat. No. 6,538,044). The compositions described in the '044 patent are fragranced lacquer coatings that are non-aqueous and are not based on urethane chemistry. The '555 publication describes solvent-based compositions useful as primers. The compositions therein are non-aqueous and require wiping of the coating with an organic solvent following exposure to UV radiation and before sanding of the coated part.
U.S. Pat. No. 6,559,225 describes an aqueous polyurethane dispersion for use in lacquers and coatings. The '225 patent does not describe UV curing, and hints that the dispersions described therein can be combined with radiation-curable binders (column 5, lines 17-20). U.S. Pat. No. 6,579,932 describes an aqueous coating composition which is a mixture of a polyurethane/acrylate hybrid dispersion and a polyurethane resin with oxidative drying groups. The '932 patent does not describe UV curing.
Aqueous radiation curable dispersions are also known (see, e.g., U.S. Pat. Nos. 5,362,773, 6,011,078, 6,479,577, 6,521,702 and 6,541,536).
Non-aqueous, radiation curable compositions are also known. WO 01/74499 describes a primer composition containing one or more compounds containing two or more ethylenically unsaturated polymerizable groups per molecule. Among the compounds described are epoxy acrylates, urethane acrylates and unsaturated polyesters. The compositions described therein also require significant amounts of relatively low molecular weight materials (such as ethylhexyl acrylate and isobornyl methacrylate). The '499 publication indicates that the compositions described therein can be cured with UV radiation using UV lamps having a UV-B:UV-A ratio of 1:1 or less and substantially no UV-C content. As in the '555 publication noted above, the compositions require wiping of the coating with an organic solvent following exposure to UV radiation and before sanding of the coated part. Similar compositions are described in published U.S. patent applications US 2003/0045598 and US2003/045596, and U.S. Pat. Nos. 4,937,173, 5,013,631, 5,213,875 and 6,509,389.
Radiation curable urethane acrylates are also described in U.S. Pat. Nos. 4,380,604, 6,232,360, 6,753,394 and 6,790,485. Finally, radiation curable epoxy acrylates are described in U.S. Pat. Nos. 5,726,255, 5,756,829, 6,359,082 and RE 37,448.
Compositions have now been discovered which can be cured with radiation having a wavelength of at least 300 nm and preferably from 320 nm to 450 nm.